The horizontal deflection of the electron beam of a television picture tube is provided by means of a timebase circuit which generates a sawtooth waveform.
It is well known that the use of a constant amplitude sawtooth waveform would result in the resulting picture suffering pincushion distorion.
This distortion is related to the vertical position of the electron beam and therefore a correction derived from the vertical timebase wavefrom needs to be superimposed onto the horizontal timebase waveform.
It is known to generate the correction waveform by components located in the vertical drive circuit. Typically a large capacitor is connected in series with the vertical deflection coils to generate a parabolic waveform by integrating the vertical ramp current.
A problem with this known circuit for providing the correction waveform is that the resulting correction current is not adjustable. The amount of correction can only be changed by changing external vertical drive components and in general will not be symmetrical about the centre of the tube. Also the large value capacitior has a very poor tolerance typically +50%, -25%.
The present invention seeks to provide a timebase circuit in which the above mentioned disadvantages of the known circuit are mitigated.